I'm not sure what I think about the question. I think it can largely depend on the player's preferred play style.
However I think lethality adds something to the game.
I was challenged by one of my player's after their group made a fatal blunder and were on the verge of a TPK and I pulled a punch so they were able to scrape through and win in the end. (In my defence, I had just bought eveyone a mini of their characters as a gift, and spent 8 consecutive evenings preparing for what lay ahead of them)
His comment was basically: 'Love your game, but what it lacks is deadly consecuences'.
I have made an effort to not be so soft over the last three sessions. Noone has died, but the encounters have been epic. That same player survived instant death 3 times, and all i needed was 9 or more to drag his immobilized body off through a dark portal (instant death). I rolled 8, 4 and 7...
In the last session, the completely erroneous instructions given by the group's dwarven cleric (third critical failure during an important skill challenge) lead to infuriating the semi-summoned manifestation of a demi-god who is now hell bent on bringing his temple down upon their heads. (The same thing that almost dragged the player's barbarian off to instant death - reskinned Thing in the Portal from KotS if anyone is curious)
The player's couldn't believe they had failed so irrevocably. That their options seemed to be flee or die. They haven't stopped talking about it. The Dwarf's player came to me today and informed me she plans to stay and try and redeem herself, even if it means certain death! he said she hadn't been able to sleep all night, running it all over in her head again and again. That's awesome! (not that she hadn't sleep, but that it had made such an impact on her)
I guess the neutrality combined with the complete disposition to kill adds something to the experience. I haven't killed anyone but the players perceive the change in disposition. It has them jumpy, nervous, excited. There is a buzz.
I have no interest in making combat so lethal it disuades players from engaging in it. I have several players that love combat. But it should be a risky and deadly enterprise. I love my players to resolve things outside of combat. But I like a mix of both. Puts spice in the blood. The threat should be a real threat. I'm not sure if this adds to roleplaying (my group varies from those that love it to those that dread it), but it adds something. It just shouldn't be sooooo lethal that it's not fun.
Anyway, just my two cents
